Method and arrangement for protecting correed matrix contacts

ABSTRACT

An arrangement and method for protecting the correed matrix contacts of a matrix, when connecting two sections of cabling together, including a near ground potential source which is coupled to the cabling at the beginning of disconnect before the connection is released, to discharge the inherent capacitance of the cabling.

United States Patent [191 Mila 3,828,315 Aug. 6, 1974 METHOD ANDARRANGEMENT FOR PROTECTING CORREED MATRIX CONTACTS SERVICE ClRCU/T E I3,713,103 l/l973 Risky 340/166 R Primary ExaminerDnald J. Yusko 7]ABSTRACT An arrangement and method for protecting the correed matrixcontacts of a matrix, when connecting two sections of cabling together,including a near ground potential source which is coupled to the cablingat the beginning of disconnect before the connection is released, todischarge the inherent capacitance of the cabling.

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METHOD AND ARRANGEMENT FOR PROTECTING CORREED MATRIX CONTACTS Thisinvention relates to a common control communication switching systemand, more particularly, to an improved centralized automatic messageaccounting system. More particularly still, it relates to a method andarrangement for; protecting the correed matrix contacts of a matrix usedin such types of systems.

In the hereinafter described centralized automatic message accountingsystem, a'correed matrix is used to connect incoming trunks to theservice circuits during call set up. Because of the inherentcharacteristics of a dry reed switch, care must be taken to protect itscontacts which arevery susceptible to damage due to large currentsswitched when sections of cable connected to each side of the'reedswitch and having different potentials on them are connected together.In this system, as well as other similar common control communicationswitching systems, such cabling cannot be avoided due to its physicalsize.

Accordingly, it is an object of the present invention to provide animprovedarrangement and method for protecting correed contacts, whenconnecting two sections of cabling together.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to'each of the others and theapparatus embodying features of construction, combination'of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

For a fuller understanding .of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings, in which:

FIG. 1 is a block diagram schematic of the centralized automatic messageaccounting system; and

FIGS. 2a and 2b show a schematic of a portion of one of the servicecircuits of the system.

Similar reference characters refer to similar parts throughout theseveral views of the drawings.

DESCRIPTION OF THE INVENTION Referring now to the drawings, in FIG. 1the centralized automatic message accounting system is illustrated inblock diagram, and the functions of the principal equipment elements canbe generally described as follows. The trunks 10, which may be eithermultifrequency (MF) trunks or dial pulse (DP) trunks, provide aninterface between the originating office, the toll switching system, themarker 11, the switching network 12, and the billing unit 14. Theswitching network 12 consists of three stages of matrix switchingequipment between its inlets and outlets. A suitable distribution oflinks between matrices are provided to insure that every inlet has fullaccess to every outlet for any given size of the switching network. Thethree stages, which consist of A, B and C crosspoint matrices, areinterconnected by AB and BC links. The network provides a minimum of 80inlets, up to a maximum of 2,000 inlets and 80 outlets. Each inletextends into an A matrix and is defined by an inlet address. Each outletextends from a C matrix to a terminal and is defined by an outletaddress.

Each full size network is divided into a maximum of 25 trunk grids onthe inlet side of the network and a service grid with a maximum of 16arrays on the outlet side of the network. The trunk grids and servicegrid within the networks are interconnected by the BC link sets of 16links per set. Each MF trunk grid is provided for inlets. Each DP trunkgrid is provided for 40 inlets. The service grid is provided for amaximum of 80 outlets. A BC link is defined as the interconnection of anoutlet of a B matrix in a trunk grid and an inlet 0 a C matrix in theservice grid.

The marker 11 is the electronic control for establishing paths throughthe electromechanical network. The marker constantly scans the trunksfor a call for service. When the marker 11 identifies a trunk with acall for service, it determines the trunk type, and establishes aphysical connection between the trunk and a proper receiver 16 in theservice circuits 15.

The trunk identity and type, along with the receiver identity, aretemporarily stored in a marker buffer 17 in the call processor 18 whichinterfaces the marker 11 and the call processor 18.

When the call processor 18 has stored all of the information transmittedfrom a receiver, it signals the marker 11 that a particular trunkrequires a sender 19. The marker identifies an available sender,establishes a physical connection from the trunk to the sender, andinforms the call processor 18 of the trunk and sender identities.

The functions of the receivers 16 are to receive MF 2/6 tones or DPsignals representing the called number, and to convert them to anelectronic 2/5 output and present them to the call processor 18. Acalling number is received by MP 2/6 tones only. The receivers will alsoaccept commands from the call processor 18, and interface with the ONItrunks 20.

The function of the MF senders are to accept commands from the callprocessor 18, convert them to MF 2/6 tones and send them to the tollswitch.

The call processor 18 provides call processing control and, in addition,provides temporary storage of the called and calling telephone numbers,the identity of the trunk which is being used to handle the call, andother necessary information. This information forms part of the initialentry for billing purposes in a multientry system. Once this informationis passed to the billing unit 14, where a complete initial entry isformated, the call will be forwarded to the toll switch for routing.

The call processor 18 consists of the marker buffer 17 and a callprocessor controller 21. Thee are 77 call stores in the call processor18, each call store handling one call at a time. The call processor 18operates on the 77 call stores on a time-shared basis. Each call storehas a unique time slot, and the access time for all 77 call stores isequal to 39.4 MS, plus or minus 1 percent.

The marker bufier 17 is the electronic interface between the marker 11and the call processor controller 21. Its primary functions are toreceive from the marker 11 the identities of the trunk, receiver orsender, and the trunk type. This information is forwarded to theappropriate call store.

The operation of the call process controller revolves around the callstore. The call store is a section of memory allocated for theprocessing of a call, and the call process controller 21 operates on the77 call stores sequentially. Each call store has eight rows and each rowconsists of 50 bits of information. The first and second rows arerepeated in rows 7 and 8, respectively. Each row consists of twophysical memory words of 26 bits per word. Twenty-five bits of each wordare used for storage of data, and the 26th bit is a parity bit.

The call processor controller 21 makes use of the information stored inthe call store to control the progress of the call. It performs digitaccumulation and the sequencing of digits to be sent. It performs fourthdigit l'blocking on a 6 or 10 digit call. It interfaces with thereceivers 16, the senders 19, the code processor 22, the billing unit14, and the marker buffer 17 to control the call.

The main purpose of the code processor 22 is to analyze call destinationcodes in order to perform screening, prefixing and code conversionoperations of a nature which are originating point dependent. This codeprocessing is peculiar to the needs of direct distance dialing (DDD)originating traffic and is not concerned with trunk selection andalternate routing, which are regular translation functions of theassociated toll switching machine. The code processor 22 is accessedonly by the call processor 18 on a demand basis.

The billing unit 14 receives and organizes the call billing data, andtranscribes it onto magnetic tape. A multi-entry tape format is used,and data is entered into tape via a tape transportoperating in acontinuous recording mode. After the calling and called directornumbers, trunk identity, and class of service information is checked andplaced in storage, the billing unit 14 is accessed by the call processcontroller 21. At this time, the call record information is transmittedinto the billing unit 14 where it is formated and subsequently recordedon magnetic tape. The initial entry will include the time. Additionalentries to the billing unit 14 contain answer and disconnectinformation.

The trunk scanner 25 is the means of conveying the various states of thetrunks to the billing unit 14. The trunk scanner 25 is connected to thetrunks by a highway extending from the billing unit 14 to each trunk.Potentials on the highway leads will indicate states in the trunks.

Each distinct entry (initial, answer, disconnect) will contain a uniqueentry identity code as an aid to the electronic data processing (EDP)equipment in consolidating the multi-entry call records into tollbilling statements. The billing unit 14 will provide the correct entryidentifier code. The magnetic tape unit 26 is comprised of the magnetictape transport and the drive, storage and control electronics requiredto read and write data from and to the 9 channel billing tape. The readfunction will allow the tape unit to be used to update the memory.

The'recorder operates in the continuous mode at a speed of 5 inches persecond, and a packing density of 800 bits per inch. Billing data isrecorded in a multientry format using a nine bit EBCDlC character(extended binary coded decimal interchange code). The memory subsystem30 serves as the temporary storage of the call record, as the permanentstorage of the code tables for the code processor 22, and as thealterable storage of the trunk status used by the trunk scanner 25.

The core memory 31 is composed of ferrite cores as the storage elements,and electronic circuits are used to energize and determine the status ofthe cores. The core memory 31 is of the random access, destructivereadout type, 26 bits per word with 16 K words.

For storage, data is presented to the core memory data registers by thedata selector 32. The address generator 33 provides the address ofcore'storage locations which activate the proper read/write circuitsrepresenting one word. The proper clear/write command allows the dataselected by the data selector 32 to be transferred to the core storageregisters for storage into the addressed core location.

For readout, the address generator 33 provides the address or corestorage location of the word which is to be read out of memory. Theproper read/restore command allows the data contained in the-word beingread out, to be presented to the read buffer 34. With a read/restorecommand, the data being read out is also returned to core memory forstorage at its previous location.

The method of operation of a typical call in the system, assuming theincoming call is via an MP trunk can be described as follows. When atrunk circuit 10 recog nizes the seizure from the originating office, itwill provide an off-hook to theoriginating office and initiate acall-for-service to the marker 11. The-marker 11 will check theequipment group and position scanners to identify the trunk that isrequesting service. Identification will result in an assignment of aunique four digit 2/5 coded equipment identity number. Through atrunk-type determination, the marker 11 determines the type of receiver16 required and a receiver/sender scanner hunts for an idle receiver 16.Having uniquely identified the trunk and receiver, the marker 11 makesthe connection through the three-stage matrix switching network 12 andrequests the marker buffer 17 for service.

The call-for-service by the marker 11 is recognized by the marker buffer17 and the equipment and receiver identities are loaded into a receiverregister of the marker buffer 17. The marker buffer 17 now scans thememory for an idle call store to be allocated for processing the call,under control of the call process controller 21. Detection of an idlecall store will cause the equipment and receiver identities to be dumpedinto the call store. At this time, the call process controller 21 willinstruct the receiver 16 to remove delay dial and the system is nowready to receive digits.

Upon receipt of a digit, the receiver 16 decodes that digit into 2/5code and times the duration of digit presentation by the calling end.Once it is ascertained that the digit is valid, it is presented to thecall processor 18 for a duration of no less than 50 milliseconds ofdigit and 50 milliseconds of interdigital pause for storage in thecalled store. After receipt of ST, the call processor controller 21 willcommand the receiver 16 to instruct the trunk circuit 10 to return anoff-hook to the calling office, and it will request the code processor22.

The code processor 22 utilizes the called number to check for EASblocking and other functions. Upon completion of the analysis, the codeprocessor 22 will send to the call processor controller 21 informationto route the call to an announcement or tone trunk, at up to four prefixdigits if required, or provide delete information pertinent to thecalled number. If the call processor controller 21 determined that thecall is an ANI call, it will receive, accumulate and store the callingnumber in the same manner as was done with the called number. After thecall process controller 21 receives ST, it will request the billing unit14 for storage of an initial entry in the billing unit memory. It willalso command the receiver 16 to drop the trunk to receiver connection.The call processor controller2l now initiates a request to the marker 11via the marker buffer 17 for a trunk to sender connection. Once themarker 11 has made the connection and has transferred the identities tothe marker buffer 17, the marker buffer will dump this information intothe appropriate call store. The call processor controller 21 nowinterrogates the sender 19 for information that delay dial has beenremoved by the routing switch (crosspoint tandem or similar). Uponreceipt of this information the call processor controller 21 willinitiate the sending of digits including KP and ST. The call processcontroller 21 will control the duration of tones and interdigital pause.After sending of ST, the call processor 18 will await the receipt of thematrix release signal from the sender l9. Receipt of this signal willindicate that the call has been dropped. At this time, the sender andcall store are returned to idle, ready to process a new call.

The initial entry information when dumped from the call store isorganized into the proper format and stored in the billing unit memory.Eventually, the call answer and disconnect entries will also be storedin the billing unit memory. The initial entry will consist ofapproximately 40 characters and trunk scanner 25 entries for answer ordisconnectcontain approximately 20 characters. These entries will betemporarily stored in the billing unit memory until a sufficient numberhave been accumulated to comprise one data block of 1,370 characters.Once the billing u'nit memory is filled, the magnetic tape unit 26 iscalled and the contents of the billing unit memory is recorded onto themagnetic tape.

The final result of actions taken by the system on a valid call will bea permanent record of billing information stored on magnetic tape inmulti-entry format consisting of initial, answer, and disconnect orforced disconnect entries.

Answer timing, force disconnect timing and other timing functions suchas, for example, a grace period timing interval on answer, in thepresent system, are provided by the trunk timers. These trunk timers arememory timers, and an individual timer is provided for each trunk in atrunk scanner memory which comprises a status section and a testsection.

The status section contains one word per ticketed trunk. Each wordcontains status, instruction, timing and sequence information. Thestatus section also provides one word per trunk group which contains theequipment group number, and an equipment position tens word thatidentifies the frame. A fully equipped status section requires 2,761words of memory representing 2,000 trunks spread over 60 groups plus astatus section start word. As each status word is read from memory, itis stored in a trunk scanner read buffer (not shown). The instruction isread by a scanner control to identify the contents of the word. Thescanner control logic acts upon the timing, sequence and statusinformation, and returns the updated word to the trunk scanner memoryand it is written into it for use during the next scanner cycle.

The test section contains a maximum of 83 words: a start word, a lastprogrammed word, 18 delay words, two driver test words, one end-testword and one word for each equipment group. The start test word causes ascan point test to begin. Thedelay words allow time for; scan pointfilters to charge before the trunk groups are scanned, with the delaywords containing only instructional data. The equipment group wordscontain a two digit equipment group identity and five trunk frameequipped bits. The trunk frame equipped bits (one per frame) indicateswhether or not a frame exists in the position identified by its assignedbit. The delay words following the equipment group allow the scan pointfilters to recharge before the status section of memory is accessedagain for normal scanning. The Last Program word inhibits read and writein the trunk scanner memory until a trunk scanner address generator hasadvanced through enough addresses to equal the scanner cycle time. Whenthe cycle time expires, the trunk scanner address generator returns tothe start of the status section of memory and normal scanningrecommences.

The trunk scanner memory and the trunk scanner read buffer are not partof the trunk scanner 25, however, the operation thereof is controlled bya scanner control which forms a part of the trunk scanner 25 of thebilling unit 14. The trunk scanner 25 maintains an updated record of thestatus of each ticketed trunk, determines from this status when abilling entry is required, and specifies the type of entry to berecorded. The entry includes the time it was initiated and theidentification of its associated trunk.

Scanning is performed sequentially, by organizing the memory in such amanner that when each word is addressed, the trunk assigned to thataddress is scanned. This causes scanning to progress in step with thetrunk scanner address generator. During the address advance interval,the next scanner word is addressed and, during the read interval, theword is read from memory and stored in the trunk scanner read buffer. Atthis point, the trunk scanner 25 determines the operations to beperformed by analyzing the word instruction.

As indicated above, scanning is performed sequentially. If all trunks inall groups are scanned in numerical sequence beginning with trunk- 0000,scanning would proceed in the following manner:

Step 1. Trunk 0000 located in frame 00 (lineup 0, column 0) in the topfile, leftmost card position would be scanned first.

Step 2. All trunks located in frame 00 and the leftmost card positionwould be scanned next from the top file to the bottom.

Step 3. Scanning advances to frame 01 (lineup 0, column l) and proceedsas in Step 2.

Step 4. Scanning proceeds as in Step 3 until frame 04 has been scanned.

Step 5. The scanner returns to frame 00 and Step 2 is repeated for thenext to leftmost card position.

Step 6. The sequence just described continues until the ten cardpositions in all five columns have been examined.

Step 7. The entire process is repeated in lineups one through five.

When a memory word instruction identifies a trunk group word, the statusreceivers are cleared to prepare for scanning the trunks specified inthe group word. The trunk group digits stored in the trunk scanner readbuffer (TSRB) are transferred into the equipment group register.

After the trunk group number is decoded, it is transformed into binarycode .decimals (BCD), processed through a l-out-of-N'check circuit, andapplied to the ACbus drivers (ACBD); The drivers activate the scan pointcircuits via the group leads and the trunk'status is'returnedtothereceivers.

A group'address applied to'the drivers causes the sta-' tus of alltrunks in one lineup and one card position and all columns to bereturned to the receivers. The group 10s digit specifies the trunk framelineup andv the group units digit identifies the card slot.

When a status wordis read from memory, it sets the previous count of atrunk timer(-TT) into the trunk timer.

If the trunk is equipped and the forced disconnect sequence equals 2(FDS=2), a request to force release the trunk is transmitted to themarker 11. If FDS does not equal 2, the present condition of theticketing contacts in the trunk is tested. If the instruction indicatesthat the trunk is in an updated condition (the trunks associated memoryword was reprogrammed) it is tested for idle. If the trunk is idle, itsinstruction'is changed to denote that it is ready for new calls. If thetrunk is not idle, no action is taken and the trunk scanner 25 proceedsto the next trunk.

If the trunk is not in the updated condition and FDS=3, the trunk istested for idle. If the trunk is idle, FDS is et to and TT is reset.

lf FDS does not equal 3 and'a match exists between the present contactstatus and the previous contact status stored in memory (bits five andsix) the FDS memory bits are inspected for a count equal to 1. If FDS=1,T1" is reset and the memory contact status is updated. If FDS does notequal 1', TT'is not reset.

During any analysis of a trunk status, a change in the contactconfiguration of a trunk is not considered valid until it has beenexamined twice.

One bit (SFT) is provided in each memory status word to indicate whetheror not a change in status of the trunk was detected during the previousscan cycle.

When a change in status is detected, SFI is set to 1. If SFT=1 on thenext cycle, the status is analyzed and SFI is set to 0.

If a mismatch exists between the present contact condition and thatpreviously stored in memory, the status has changed and a detailedexamination of the status is started.

If CT=l the trunk is busy and so the previous condition of the contactis inspected. If the trunk previously was idle, CM=0. Before continuingthe analysis, it must be determined if this is the first indication ofchange in the trunk status by examining the second look bit (SFI). IfSFT=0,it is set to equal 1, and the analysis of this trunk status isdiscontinued until the next scanner cycle. If SFT=l the memory status isupdated and SFT is set to equal 0.

If CT=l, the trunk is cut through and CM is inspected to determine ifthe memory status was updated. If CM=l, the GT contact status mustdiffer from GM since it was already determined that a mismatch exists.If GT=0, answer has not occurred. If GT=1, and this condition existedduring the previous scan cycle, SPT=1 also. If these conditions are trueand FDS does not equal 1, TT is advanced and answer timing begins. Ifthese conditions persist for eight scanner cycles (approximately 1second), answer is confirmed and an entry will be stored in the trunkscanner formater (TSF). If answer is aborted (possibly hookswitch fumble) before the 1 second answer time (time is adjustable) expires, TTremains at its last count. When the answer condition returns, answertimingcontinues from the last 'IT count. Thus, answer timing iscumulative.

After ananswer entry is stored, which includes the TT count, T1" isreset, SPT is set to 0, andthe new contact status is written intomemory. f I

If a mismatch exists and CT=0, the previous state of this contactisinspected by examining bit in the trunk scannerread buffer (TSRB). IfCM=l, the state of the terminating end of the trunk is tested. If GT=1,then the condition of the trunk has just changed from 'answer todisconnect. If this condition existed during the previous scan cycle,SFT=1 and a disconnect entry is If a mismatch exists and the originatingend of a trun is, not released, both CT and CM equals 1.. If vGT=0 afterthe previous scan. cycle, FDS is tested. If this change just occurred, FDS does not equall. Since FDS does not equal 1, it will be set equal tol and T1" will reset. FDS=1 indicates that forced disconnect timing isin progress.

While the conditions just described exist, i.e., mismatch, CT=l, CM=l,GT=0 and FDS=1,' TT will ad vance 1 count during each scanner cycle, ifone half second has elapsed since the last scan cycle. Tlwill continueto advance until it reaches a count of 20 (approximately 10 seconds)when a'forced disconnect entry will be stored in' the TSF.

When the entry is stored, FDS is set atZ indcating that the trunk is tobe force released. After tne'entry is stored, which includes the TIcount, TT isreset, SFT is set to 0, and the new status is written intomemory.

After the status and test sections of the memory have been accessed, theLast Program word-is read from memory and stored in the trunk scannerread buffer. This word causes read/write in the trunk scanner portion ofmemory to be inhibited and deactivates the scan point test. The trunkscanner address generator will continue to advance, however, untilsufficient words have been addressed to account for one scan cycle. Whena predetermined address, the Last Address, is reached, block read/writeis removed and the address generator returns to the Start Address (FirstProgram Word) of the scanner memory.

From the above description of the centralized automatic messageaccounting system, it can be seen that the trunks 10 are connected tothe service circuits 15, including the receivers 16 and the senders 19,by the switching network 12 which is a correed matrix. As furtherindicated above, the contacts of the correed matrix are very susceptibleto damage due to large currents switched when sections of cablesconnected to each side of the dry reed switches and having differentpotentials on them are connected together.

In accordance with the present invention, the switching network 12 ormatrix is protected by circuitry within the service circuits 15 whichsenses the wetting current provided through the matrix by the trunks.This current is approximately 2-3 milliamps and is placed on the matrix12 via 15K resistors in the trunks 10. It is electronically sensed witha transistor sensing circuit in each of the service circuits 15 andcauses a potential which is close to ground to be switched on to certainones of the leads connecting the service circuits 15 to the matrix 12,before the matrix path can be released. This potential discharges theinherent capacitance of the cabling in the matrix and thereby insuresthat the use of this cabling for succeeding connections will not causethe systems marker to connect two sections of cable having differentstore charges on them. This arrangement, as more fully set forth below,removes the requirement for the common control circuitry of the systemto constantly monitor both the trunks l and the service circuits l andsense when the path is to be released. Furthermore, it also removes therequirement for additional external circuitry to access the paths anddischarge them remotely.

More particularly, in FIG. 2, one of the service circuits 15, a receiver16, is illustrated and it can be seen that it as well as each of theother service circuits is connected to the C stage of the switching.network 12 or matrix via four leads (T, R, C and H) which are extendedvia the matrix 12 to a trunk 10. Two other leads (not shown) are used bythe marker 1 1 for circuit identification and busy/idle status of thereceiver 16.

During call setup, the H relays in a trunk and a receiver 16 both areoperated by a 50 volt potential extended through these relays to aground potential, in the generally well-known fashion. The H relay inthe receiver 16, upon operating, extends ground through its normallyopen contact H1 to the base of transistor O4, to cause transistor O4 toturn ON. The transistor O4, in turning ON, causes transistor O5 to turnOFF. Transistor O5 in turning OFF, removes the 50 volts on the base oftransistor Q6, thus causing it to turn ON. This action, in turn, causesthe 49 volts on the emitter of transistor O6 to be coupled to the baseof the transistor Q12 and cause transistor Q12 to turn ON. Thetransistors Q6 and Q12 being ON, prevent the transistors Q10 and Q13from turning ON, by placing the near ground potential (-1 volt) on thebases of these two transistors. These transistors Q10 and Q13 are heldOFF, to prevent contact stagger on the correed relay HC from causingthem to operate or turn ON, during call setup.

The marker 11 during the first stages of call set up applies a groundpotential to the lead AST (or EST), and this ground potential keeps thetransistor Q9 turned ON and the transistor Q41 turned OFF. However,during switch through of the trunks and receivers, this ground potentialis removed and the transistors Q9 and 041 are caused to turn OFF and ON,respectively. When the transistor Q41 turns ON, the correed relay HC andthe relay HD both are caused to operate. The correed relay HC operatesvia the near ground (-3 volts) potential on the emitter of thetransistor 041 being extended through it to the 50 volts on the emitterof transistor Q4. The relay HD operates via the same near groundpotential on the emitter of transistor Q41 being extended through it andthe diode CR89 to the -50 volt potential, when the contact HC2 of thecorreed relay l-IC operates. When these relays HC and HD operate, thereceiver 16 is connected through the matrix 12 to the trunk 10. During acall setup, as described above, the marker 11 performs a continuity testand a foreign potential check before connecting the re.-

ceiver 16 to the trunk 10, as more fully described in a copendingapplication Ser. No. 357,310; filed by Melvin A. Jacobs on May 4, 1973and assigned to the same assignee as is the present application.Reference may be made to this copending application for a more completedescription of the systems operation during call setup, however, for thepurposes of the present invention, it is only necessary to generallystate that prior to the receiver 16 being connected through the matrix12 to the trunk 10 the above described operations occur in the receiver16.

The attached trunk 10 provides volts on the T and R leads through the15K ohm resistors R5 and R6, which 50 volts is extended through thecontacts HC4 and HC5 of relay HC and the two resistors R12 and R13. This50 volt potential is sensed by the transistor Q14, via the resistors R12and R13, and the transistor Q14 is turned ON by it. The transistor Q14,in turning ON, provides a 3 volt signal through either the externalrelay contacts 88 or the timer 89, to the base of transistor Q36,causing transistor 36 to turn ON. The external relay contacts 88 and thetimer 89 are for purposes separate from the present invention and henceare not more fully discussed herein. When transistor Q36 turns ON, itcouples the 49 volt potential on its emitter to the base of transistorQ12, to hold it turned ON and this allows the marker 11 to turn OFFtransistor Q6. The transistor Q6 initially was operated by the marker11, to prevent transistors Q10 and Q13 from turning ON, due to contactstagger on the correed relay HC.

Now, when the trunk 10 begins to disconnect the -50 volts on the leads Tand R is removed. This lack of current flow causes transistor Q14 toturn OFF, which action, in turn, causes transistors Q36 and Q12 to turnOFF. The transistor Q12 in turning OFF, allows the -50 volts throughcontact HC2 of the correed relay BC to reach the transistors Q10 andQ13. This potential on the bases of these two transistors causes them toturn ON. The transistor Q10 in turning ON extends the -2 volts on itsemitter to the C lead, and the transistor Q13 in turning ON extends the--2 volts on its emitter to the T and R leads. This 2 volts on theseleads T, R and C discharges the connected cable capacitance to such alow level that a future connection with these cables will not find asufficient voltage difference to cause destructive currents to flowthrough the correed contacts in the switching network or matrix 12.

Accordingly, from the above description, it can be seen that a potentialclose to ground is switched onto the leads T, R and C of the matrix 12before the matrix path is released, to discharge the inherentcapacitance of the cabling in the matrix. By doing so, the use of thiscabling for succeeding connections will not cause the marker 11 toconnect two sections of cable having different store charges on themwhich could damage the contacts of the correed relays in the matrix 12.

It will thus be seen that the objects set forth above among those madeapparent from the preceding description, are efficiently attained andcertain changes may be made in carrying out the above method and in theconstruction set forth. Accordingly, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

Now that the invention has been described, what is claimed as new anddesired to be secured by Letters Patent is:

1. In a common control communication switching system including aplurality of trunks connectable through a switching network to any oneof a plurality of service circuits under the control of a marker, saidswitching network being formed with cabling and correed relays havingcontacts for establishing such connections, said trunks once aconnection is established coupling a wetting current through saidswitching network to said service circuits and removing the same at thebeginning'of disconnect, an arrangement within each of said servicecircuits for discharging the inherent capacitance of the cabling in theswitching network by means of which a connection is established toprotect the contacts of the correed relays from damage comprising apotential source, switching means for coupling said potential source tosaid cabling to discharge said inherent capacitance of said cabling,sensing means for sensing said wetting current and being operable torender said switch means inoperable as long as said wetting current issensed and to operate said switch means to couple said potential sourceto said cabling upon sensing the absence of said wetting current,whereby said potential source is coupled to said cabling at thebeginning of disconnect when said trunk removes said wetting current andbefore the-connection is released.

2. The arrangement of claim 1, wherein said sensing means comprises atransistor sensing circuit in each of said service circuits.

3. The arrangement of claim 1, wherein said switch means comprises atransistor, said potential source being coupled to the emitter of saidtransistor and the collector thereof being coupled to said cabling,whereby said transistor upon being rendered conductive extends saidpotential source to said cabling, said transistor being renderedconductive by said sensing means when the latter senses the absence ofsaid wetting current.

4. The arrangement of claim 1, wherein said sensing means comprises atransistor sensing circuit in each of said service circuits, saidtransistor sensing circuits each comprising a pair of resistorsconnected in series with the cabling through which said wetting currentflows and a transistor having a base connected to the juncture betweensaid pair of resistors and rendered conductive when said wetting currentflows through said pair of resistors, said transistor being coupledtoand'rendering said switch means inoperable when it is conductive.

i l III I

1. In a common control communication switching system including aplurality of trunks connectable through a switching network to any oneof a plurality of service circuits under the control of a marker, saidswitching network being formed with cabling and correed relays havingcontacts for establishing such connections, said trunks once aconnection is established coupling a wetting current through saidswitching network to said service circuits and removing the same at thebeginning of disconnect, an arrangement within each of said servicecircuits for discharging the inherent capacitance of the cabling in theswitching network by means of which a connection is established toprotect the contacts of the correed relays from damage comprising apotential source, switching means for coupling said potential source tosaid cabling to discharge said inherent capacitance of said cabling,sensing means for sensing said wetting current and being operable torender said switch means inoperable as long as said wetting current issensed and to operate said switch means to couple said potential sourceto said cabling upon sensing the absence of said wetting current,whereby said potential source is coupled to said cabling at thebeginning of disconnect when said trunk removes said wetting current andbefore the connection is released.
 2. The arrangement of claim 1,wherein said sensing means comprises a transistor sensing circuit ineach of said service circuits.
 3. The arrangement of claim 1, whereinsaid switch means comprises a transistor, said potential source beingcoupled to the emitter of said transistor and the collector thereofbeing coupled to said cabling, whereby said transistor upon beingrendered conductive extends said potential source to said cabling, saidtransistor being rendered conductive by said sensing means when thelatter senses the absence of said wetting current.
 4. The arrangement ofclaim 1, wherein said sensing means comprises a transistor sensingcircuit in each of said service circuits, said transistor sensingcircuits each comprising a pair of resistors connected in series withthe cabling through which said wetting current flows and a transistorhaving a base connected to the juncture between said pair of resistorsand rendered conductive when said wetting current flows through saidpair of resistors, said transistor being coupled to and rendering saidswitch means inoperable when it is conductive.